1. Field of the Invention
This invention relates to contour or line correcting apparatus for video signals which emphasize signals corresponding to contours or lines included in the video signals.
2. Description of the Prior Art
Up to the present, various kinds of systems to emphasize the contours of video signals have been presented. Among them, one particularly practical one is the system in which a secondary differentiated waveform of a video signal is inverted in phase and added to the original video signal. Examples of waveforms according to this conventional technique will be described referring to FIG. 1. A primary differentiated waveform (b) is obtained by differentiating the original video signal (a). A secondary differentiated waveform (c) is obtained by further differentiating the wave (b) and then phase inverting it. By adding the signal (c) to the original signal (a), a video signal (d) having a corrected contour or border line is obtained. However, that system has the drawback that, for example, the border line correction may be excessive, since the differential coefficients increase when the leading and trailing edges of the input video signal are steep. Moreover, since the differentiating circuit may generally be composed of a resistor and a capacitor, the differentiation characteristic becomes approximate, differentiation is done imperfectly and, when the leading and trailing edges of the video signal (a) are steep as shown in FIG. 3, the amplitude of the primary differentiated waveform (b) approaches the amplitude of the original signal (a). Since the trailing edge characteristic of the primary differentiated waveform is based on the time constant of the differentiating circuit, there exists a drawback that the border line correction is done asymmetrically.